We are studying mechanisms of contractility associated with smooth and cardiac muscle myosin light chains. Using reverse phase HPLC, we have purified the 20,000 MW phosphorylatable light chain from a number of different sources. We plan to develop an assay for myosin light chain phosphorylation in cardiac muscle treated with various inotropes. Finally, we are assaying in vitro inhibition of myosin kinase activity by vasolidators.